1. Technical Field
The present invention relates to processes for manufacturing machine components such as carriers for a planetary gear system that is included in an automatic transmission of an automobile (hereinafter called a “planetary carrier”) by a powdered metallurgical method. Specifically, the present invention relates to a process for manufacturing composite sintered machine components in which a compact (an inner member) having plural pillars and another compact (an outer member) having holes corresponding to the pillars are tightly fitted and are sintered so as to bond each other.
2. Background Art
Although planetary carriers differ in design according to the type of transmission, they usually comprise a cylindrical drum, flanges formed at both ends or at the middle of the drum, and a center shaft hole into which a shaft of a transmission is inserted. Generally, the drum is formed with plural openings for holding planetary gears (not shown in the figure). FIG. 1 shows an example of such a planetary carrier, and each of the plural (in this case, three) openings 11 formed on a drum 10 is rotatably mounted with a planetary gear (not shown in the figure). The planetary gear is engaged with a sun gear of a shaft (not shown in the figure) inserted into a center shaft hole 12 of the drum 10 at the inner side of the drum 10, and it is engaged with a ring gear (not shown in the figure) at the outer side of the drum 10. Flanges 20 and 25 are formed at the upper end and the lower end of the drum 10, and the flange 20 in the upper side of the figure is formed with spur teeth 21 for transmitting a torque. Moreover, a boss 23 is concentrically formed on the upper surface of the upper flange 20, and the boss 23 is formed with a spline 24 for engaging a clutch system (not shown in the figure).
Thus, since a planetary carrier has such a complicated structure, if it is mass-produced by machining process such as cutting, great number of processing steps are required, whereby there are disadvantages in cost and accuracy of shape and size. Therefore, planetary carriers are usually manufactured by a powdered metallurgical method that is suitable for manufacturing products uniformly in large quantities; however, in the case of planetary carriers having openings forming undercuts, which are provided on a drum, it is difficult to form them unitarily in a die.
As a method developed to solve these problems, a required shape is divided into several portions, and after the portions are individually formed and sintered, they are combined to form the required shape. For convenience of explanation, a planetary carrier will be described based on a schematic shape shown in FIG. 2 hereinafter. The planetary carrier shown in FIG. 2 has a simple flange 20 at the upper end and a simple flange 25 at the lower end on a cylindrical drum 10, and it has three openings 11 at equal intervals in the circumferential direction of the drum 10. In the planetary carrier shown in FIG. 1, the spur teeth 21 and the boss 23 of the flange 20 are omitted. In order to form the planetary carrier having such shape by die forming, the planetary carrier is divided into two portions by separating one flange 20 (25) from the drum 10.
Specifically, as shown in FIGS. 3A to 3F, a planetary carrier is divided into a disk-shaped member 30 (corresponding to the flange 20 in FIG. 2) having a center shaft hole 31 and a body member 40, and the disk-shaped member 30 and the body member 40 are individually formed and sintered so as to make two portions. Then, the sintered disk-shaped member 30 and the sintered body member 40 are mated and bonded by brazing at the divided surfaces. FIG. 3A is a top view of the disk-shaped member 30, FIG. 3B is a longitudinal sectional view of the disk-shaped member 30, FIG. 3C is a top view of the body member 40, FIG. 3D is a longitudinal sectional view of the body member 40, FIG. 3E shows a condition in which the disk-shaped member 30 and the body member 40 are bonded, that is, it is a top view showing a condition shown in FIG. 2, and FIG. 3F is a longitudinal sectional view of the condition shown in FIG. 3E. In this case, the drum of the body member 40 has relatively large openings, and the appearance thereof may be described as “three fan-shaped pillars”. Therefore, the drum will be called plural (three) pillars 42 hereinafter. That is, the body member 40 has a shape in which a disk-shaped portion 47 having a center shaft hole 41 is integrally fixed to ends of the plural pillars 42.
When the disk-shaped member 30 and the body member 40 are brazed, since a liquid phase is generated at the bonding surface, the centers thereof may not be aligned (the axes thereof may not be aligned), and the phases thereof may be misaligned (they may be misaligned in circumferential direction), whereby the accuracy of the products tends to be decreased. Moreover, the bonding strength of the disk-shaped member 30 and the body member 40 mainly depends on the strength of the brazing metal, whereby it is difficult to obtain the required level of strength.
Methods of improvement have been suggested to deal with the above problems and are disclosed in Japanese Patents Nos. 1427539 corresponding to U.S. Pat. No. 4,503,009 (patent document 1), 1781330 (patent document 2), and 3495264 corresponding to U.S. Pat. No. 6,120,727, GB. Patent No. 2343682, and DE. Patent No. 19944522 (patent document 3). The methods of improvement employ a technique in which a hole provided in one compact is tightly fitted with a pillar portion provided at another compact, and these are sintered so as to bond together. That is, as shown in FIGS. 4A to 4F, a body member 40 is a compact (inner member) in which fan-shaped pillars 42 are integrally formed, and a disk-shaped member 30 is a compact (outer member) in which holes 32 corresponding to the shape of the pillars 42 of the body member 40 are formed in connection with a center shaft hole 31. Then, the body member 40 and the disk-shaped member 30 are sintered in a condition in which the pillars 42 of the body portion 40 are tightly fitted to the holes 32 of the disk-shaped portion 30. In this case, they are sintered in such a way that the amount of thermal expansion of the body member 40 is set to be greater than the amount of thermal expansion of the disk-shaped member 30 in a high temperature range (diffusion temperature range of additive ingredients) in sintering, thereby obtaining a sintered component having a predetermined shape. FIG. 4A is a top view of the disk-shaped member 30, FIG. 4B is a longitudinal sectional view of the disk-shaped member 30, FIG. 4C is a top view of the body member 40, FIG. 4D is a longitudinal sectional view of the body member 40, FIG. 4E is a top view showing a condition in which the pillars 42 of the body member 40 are tightly fitted to the holes 32 of the disk-shaped member 30, and FIG. 4F is a longitudinal sectional view showing the condition shown in FIG. 4E.
In order to produce the above-described condition in which the amount of thermal expansion of the inner member (body member 40) is greater than the amount of thermal expansion of the outer member (disk-shaped member 30) in the high temperature range during sintering, in the patent document 1, carbon is included in an inner member as an essential ingredient at an amount greater than that of an outer member by at least 0.2 mass %. In the patent document 2, an iron powder forms an outer member, and 5 to 10% of the iron powder is made from a carbonyl iron powder. In the patent document 3, a zinc stearate is used as a powdered lubricant only in an inner member, and it is sintered in a carburizing atmosphere so that the amount of the thermal expansion of the inner member is increased.
According to the methods, the above-mentioned misalignments of the centers and the phases do not occur, but the bonding surfaces of the inner member and the outer member tend to be insufficiently bonded each other, and the required level of the bonding strength may not be obtained. The reason for this is described hereinafter. That is, in the case of the above method in which the pillar (which approaches the inner side by tightly fitting) is tightly fitted to the hole (which approaches the outer side by tightly fitting) of a compact, if the contacting surface thereof is a tightly fitted cylindrical surface, and the amount of thermal expansion of the pillar side (inner side) is greater than that of the hole side (outer side), the entire surface of the contacting surface is tightly contacted, whereby the pillar and the hole are bonded by diffusion. On the other hand, in the case of the planetary carrier shown in FIGS. 4A to 4F, the contacting surface of the disk-shaped member 30 and the body member 40, that is, the contacting surface of the pillars 42 and the inner surface of the holes 32 into which the pillars 42 are inserted, is not completely closed, and the contacting surface is open to the center shaft hole 31. Therefore, even though the amount of thermal expansion of the body member 40 is set to be relatively greater than that of the disk-shaped member 30 as in the methods disclosed in the patent documents 1 to 3, pressure due to the expansion of the pillars 42 impinges on the side of the center shaft hole 31, whereby the contacting surface of the disk-shaped member 30 and the body member 40 may not tightly contact, and the bonding strength is decreased.
Furthermore, a method is disclosed in Japanese Patent No. 3833502 (patent document 4). As shown in FIGS. 5A to 5F, both sides 45, which are the sides of the pillars 42 provided to the body member 40 (inner member), are modified so as to have a refractile surface (stepped shape), and the outline of the holes 32 provided to the disk-shaped member 30 (outer member) is modified so as to have a shape corresponding to the sides of the pillars 42 so as to secure the bonding strength. According to that shape, the effect of strain based on the difference of the amount of thermal expansion occurring at the bonding surface of the pillars 42 and the inner surface of the holes 32 during sintering is decreased, and the expansion pressure of the pillars is prevented from escaping to the side of the center shaft hole 31 because the pillars 42 are thin at the bent portion, whereby the bonding strength is secured.
The technique disclosed in the patent document 4 is an elaboration of the technique disclosed in the patent documents 1 to 3, and it is based on a condition in which the amount of thermal expansion of the body member 40 is greater than that of the disk-shaped member 30. In this case, not only the pillars 42, but also the entire body member 40 can expand, and even when the expansion of the pillars 42 is restricted by the holes 32 of the disk-shaped member 30, a deflection may occur because the remaining portion expands, and the degree of parallelization of the disk-shaped member 30 and the body member 40 is thereby lost.
Since the planetary carrier is formed by arranging flanges at both ends of the pillars, if the degree of parallelization is lost in this way, the shape is difficult to correct by applying pressure again. Therefore, deflection that occurred during sintering and bonding will be a disadvantage in manufacturing. Moreover, the disk-shaped member 30 has a thin portion 38 between an outer periphery 37 and the hole 32 of the disk-shaped member 30 shown in FIGS. 4A to 4F and FIGS. 5A to 5F, and the thin portion 38 deforms according to the expansion of the body member 40, especially, the pillars 42, whereby there are disadvantages in which the degree of circularity of the sintered disk-shaped member 30 (in the planetary carrier shown in FIG. 1, the dimensional accuracy of the teeth) is inferior, and fracture may occur at the thin portion 38.